c++: missing SFINAE during alias CTAD [PR115296]

During the alias CTAD transformation, if substitution failed for some
guide we should just silently discard the guide.  We currently do
discard the guide, but not silently, as in the below testcase which
we diagnose forming a too-large array type when transforming the
user-defined deduction guides.

This patch fixes this by using complain=tf_none instead of
tf_warning_or_error throughout alias_ctad_tweaks.

	PR c++/115296

gcc/cp/ChangeLog:

	* pt.cc (alias_ctad_tweaks): Use complain=tf_none instead of
	tf_warning_or_error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias23.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 108e929b8ee6cd9348c0699f0a028a3cf2c90540..8cc5e21c520dc164149f667defb0b6bec11a3b18 100644 (file)
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30287,7 +30287,7 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
                          (INNERMOST_TEMPLATE_PARMS (fullatparms)));
     }
 
-  tsubst_flags_t complain = tf_warning_or_error;
+  tsubst_flags_t complain = tf_none;
   tree aguides = NULL_TREE;
   tree atparms = INNERMOST_TEMPLATE_PARMS (fullatparms);
   unsigned natparms = TREE_VEC_LENGTH (atparms);

diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias23.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias23.C
new file mode 100644 (file)
index 0000000..117212c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias23.C
@@ -0,0 +1,19 @@
+// PR c++/115296
+// { dg-do compile { target c++20 } }
+
+using size_t = decltype(sizeof(0));
+
+template<class T, size_t N = size_t(-1)>
+struct span { span(T); };
+
+template<class T, size_t N>
+span(T(&)[N]) -> span<T, N>; // { dg-bogus "array exceeds maximum" }
+
+template<class T, size_t N>
+requires (sizeof(T[N]) != 42) // { dg-bogus "array exceeds maximum" }
+span(T*) -> span<T, N>;
+
+template<class T>
+using array_view = span<T>;
+
+array_view x = 0;